1. Field of the Invention
The present invention relates to the investigation of earth formations. More particularly, the invention relates to methods and apparatus for electrically imaging the wall of a borehole.
2. State of the Art
When analyzing a hydrocarbon well, it is desirable to identify earth formation features at various borehole depths. Some of the formation features which are desirable to identify include fine beddings and facies, the heterogeneity of carbonate deposits and the structure of fractures.
The detection of beddings involves detecting shaly-sand sequences where the shales establish a basal contact for each sequence. Facies identification involves identifying the lithology between basal contacts. The analysis of carbonates involves detecting non-homogenous features such as those due to irregular cementation, variations in the pore sizes, small scale lithology changes, etc. Fractures play a major role in the flow characteristics of reservoir rock. Therefore, the measuring or detecting of fractures, determining their orientations, density, height, vertical and lateral continuity is highly desirable.
Co-owned U.S. Pat. No. 4,468,623 to Gianzero et al. (the '623 patent) discloses an earth formation investigating tool which can detect borehole wall features which are only millimeters in size. As shown in prior art FIGS. 1 and 2, the tool 10 includes an array 12 of small survey electrodes (buttons) 14a-14l mounted on a conductive pad 16 which is pressed against the borehole wall 18. A constant current source is coupled to each button such that current flows out of each button 14 into the adjoining formation, perpendicular to the borehole wall 18 as illustrated in FIG. 1 by the arrows E.sub.1, E.sub.2. The current returns to an electrode (not shown) which is located at or near the surface, or on another part of the tool 10. The individual button currents are monitored and recorded (by an uphole processor 20) as the tool 10 is moved through the borehole. The measured button currents are proportional to the conductivity of the material in front of each button. The conductivities are plotted as a function of depth to form a "wiggle trace" (or log) which can be analyzed to identify formation features at the different borehole depths.
Co-owned U.S. Pat. No. 4,567,759 to Ekstrom et al. (the '759 patent) discloses a method and apparatus for producing a high resolution image from the data collected by the tool described in the '623 patent. According to the methods of the '759 patent, signals from a conductivity measuring tool are processed to compensate for conditions such as variation in tool velocity, variations in borehole environment, etc. This processing enables subsequent signal enhancements with which the signals can be displayed in a manner that approaches the character of a visual image of the borehole wall taken from inside the borehole. Since the human eye is highly perceptive, fine high resolution features of the borehole wall can be visually discerned and interpreted. Such features include minute variations in the borehole wall in both the circumferential as well as vertical directions. Features which can be discerned from the image include vugs, small stratigraphy beds with their circumferential thickness variations, small scale lithology changes, pore sizes, fractures with their density, height, vertical and lateral continuity, etc.
Further enhancements to the methods and apparatus of the '623 patent and the '759 patent are disclosed in co-owned U.S. Pat. No. 4,692,908 to Ekstrom et al.(the '908 patent). The '908 patent discloses an acoustic method and apparatus for measuring the distance between the electrode buttons and the borehole wall. This distance is likely to change as the tool is moved through the borehole. Distance measurements made according to the '908 patent are recorded and used to correct the conductivity measurements if deemed necessary.
As disclosed in the '623 patent, the size and spacing of the electrode buttons is important to obtain good resolution and signal to noise ratio. In particular, the buttons should be closely spaced for high resolution and small in area for good spatial bandwidth. If the buttons are too small, however, the signal to noise ratio (SNR) is adversely affected. This is demonstrated by analyzing the current flow through the buttons into the formation. For example, as shown in prior art FIG. 1, buttons 14a and 14b are located on opposite sides of a bed boundary B which separates beds having different resistivities R.sub.1 and R.sub.2. Assuming that the pad 16 is in perfect contact with the borehole wall 18, the electric field near the pad 16 is perpendicular to the pad face or parallel to the bed boundary B. The parallel component of the electric field is continuous across the two different media as shown by EQU E.sub.1 =E.sub.2 (1)
where E.sub.1 and E.sub.2 are the electric fields on the two sides of the bed boundary B. In an ideal case, with an infinitely long pad, the equipotential surfaces near the center of the pad are all parallel to the pad face and the electric field is constant. The current density j.sub.i flowing into each bed is proportional to the conductivity .sigma..sub.i of that bed as shown according to EQU j.sub.i =E.sigma..sub.i (2)
If the conductivity .sigma. of the formation is continuously varying, then the current density j may be expressed by EQU j=E.sigma. (3)
The electric current I.sub.b flowing into a button is therefore the integral of the current density over the area of the button according to EQU I.sub.b =.intg.j.multidot.da=E.multidot..intg..sigma.da (4)
The electric field E depends on the conductivity distribution away from the pad and is not calibrated. Therefore, the button current is not a quantitative measure of the local conductivity. However, if one computes the ratio of button currents at two nearby points, the unknown Es cancel out. Thus, the ratio of the currents passing through two nearby buttons is a quantitative measurement of the ratio of shallow conductivities. From the foregoing, it will be appreciated that the size and spacing of the electrode buttons will govern resolution and SNR, and that resolution can be increased only at the expense of decreasing SNR.
The methods and apparatus thus far described with reference to the co-owned '759 and '623 patents are known in the art as FMI.TM., a trademark of Schlumberger and an abbreviation for "formation micro imager". FMI.TM. has been widely successful in producing accurate images of boreholes when used in wells which have been drilled with water based mud (WBM). However, the FMI.TM. produces lower quality images in wells which have been drilled with oil based mud (OBM).
At the present time there are no tools available which can produce borehole images in an OBM well which are comparable in quality to the images produced by FMI.TM. in a WBM well. Despite this fact, the use of OBM in well drilling is increasingly popular.
It is believed that in an OBM well non-conductive mudcake between the conductor buttons and the borehole wall interferes with the conductivity measurements.